Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22C—FOUNDRY MOULDING
  • B22C1/00—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds
  • B22C1/16—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds characterised by the use of binding agents; Mixtures of binding agents
  • B22C1/18—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds characterised by the use of binding agents; Mixtures of binding agents of inorganic agents
  • B22C1/183—Sols, colloids or hydroxide gels
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22C—FOUNDRY MOULDING
  • B22C1/00—Compositions of refractory mould or core materials; Grain structures thereof; Chemical or physical features in the formation or manufacture of moulds

Definitions

• This invention relates to a process for producing a mold for casting using a slurry of refractory particles and colloidal alumina as binder. More particularly it relates to a process for producing a mold suitable for use as a core used in a precise casting mold cavity.
• binders In making molds, particularly cores used in molds, there are used in general water glass, clays, plastics and the like as the binder. These binders have considerable binding strength and are stable at low temperatures, but their binding strength is lowered at high temperatures at 1200° C. or higher. Particularly when a fluid metal at high temperature is cast, there arises a defect in that the core is deformed and sometimes destroyed.
• a solution of hydrolyzed ethyl silicate as a binder is disclosed, for example, in Japanese Patent Appln Kokoku (Post-Exam Publn) No. 20848/63.
• a hydrolyzed solution of ethyl silicate is mixed with refractory particles to prepare a slurry, which is filled in a pattern and gelled, followed by drying and firing.
• the pattern is sometimes dipped in water. Since an alcohol is used as binder and should be evaporated with the progress of gellation, there easily take place fine cracks in the mold in such a time. Thus, the strength of the mold is lowered by the generation of cracks.
• This invention provides a process for producing a mold for casting which comprises mixing refractory particles with a colloidal alumina solution as the binder to give a slurry, filling said slurry in a pattern (or pattern cavity) having water absorption properties, followed by drying and firing.
• the slurry obtained by kneading refractory particles and colloidal alumina showed very good fluidity and when it was filled in a pattern cavity (or a core box), it was able to reach all the corners without causing voids even if the pattern had a complicated shape. Further, it was also found that when the slurry was filled in a pattern cavity having water absorption properties and allowed to stand without stirring, the viscosity of the slurry increased gradually. This invention has been accomplished by applying these phenomena newly found to the production of molds, particularly cores.
• refractory particles and colloidal alumina are mixed and kneaded to give a slurry, which is then poured or filled in a pattern (cavity) having a predetermined shape and water absorption properties; these procedures are essential in this invention. If the slurry is not filled in a pattern (cavity) having water absorption properties, the slurry does not form a solid. In order to solidify the slurry, removal of water from the slurry is necessary. In order to remove the water, the pattern having water absorption properties should be used.
• the pattern having water absorption properties there can be used those made of plaster, synthetic resins having water absorption properties, metal plates having a large number of fine connected pores, or sintered bodies such as sintered metals or ceramics having a large number of fine connected pores, etc.
• the pore size of the pattern having water absorption properties should be smaller than the particle size of the refractory particles. Since the particle size of the refractory particles is preferably 30 ⁇ m or more, the pore size of the pattern is preferably smaller than 30 ⁇ m.
• the filling method of slurry in the pattern (cavity) there can preferably be used a vibration method wherein the pattern is vibrated, or a method wherein the slurry is filled with application of pressure.
• a vibration method wherein the pattern is vibrated
• a method wherein the slurry is filled with application of pressure.
• packing of refractory particles can be increased and molds and cores having complicated shapes can be produced with dimensional accuracy.
• the method of filling slurry with applying pressure there can be employed a method wherein a vessel containing the slurry and an opening of the pattern (cavity) (or core box) for filling the slurry are connected with a pipe, through which the slurry can pass into the pattern (cavity) by the pressure applied to the slurry surface in the vessel by means of a gas such as air.
• the pressure applied to the slurry changes depending on the fluidity of slurry but usually is 0.1 to 2 kg/cm 2 .
• the vibration method there can be employed a conventional vibrator.
• the direction of vibration may be either up-and-down or horizontal.
• refractory particles there can be used those generally used in making molds.
• the refractory material are zircon, alumina, sillimanite, quartz, mullite, magnesia, etc.
• the particle size of the refractory particles is preferably 150 ⁇ m or less. It is more preferable to use refractory particles having a smaller particle size and those having a larger particle size as a mixture thereof.
• a slurry obtained by kneading refractory particles having a smaller particle size and those having a larger particle size together with colloidal alumina is filled in the pattern (cavity) having water absorption properties, the refractory particles having a smaller particle size are gathered at the side contacting with the inner surface of the pattern (cavity), that is, the surface of the mold or core. As a result, the surface roughness of the mold (or core) is lessened and a smooth surface can be obtained. Further, since the mold contains the refractory particles having a larger particle size in larger amount in the inner portion and mechanical strength becomes smaller than that of the mold surface, destruction of the mold after casting becomes easy.
• refractory particles having a particle size of 80 ⁇ m or less it is preferable to use refractory particles having a particle size of 30 to 80 ⁇ m.
• the particle size is smaller than 30 ⁇ m, mold release characteristics become worse and the mold surface tends to become rough.
• the particle size of refractory particles having a larger particle size is preferably between 80 ⁇ m and 150 ⁇ m, more preferably 100 to 150 ⁇ m. If the particle size is too large, binding strength becomes undesirably lessened.
• the mixing ratio of refractory particles having a smaller particle size to those having a larger particle size is preferably 6:4 to 7:3 by weight.
• the colloidal alumina is a milky white viscous solution stable to inorganic acids and contains about 10% by weight of Al 2 O 3 dispersed in a liquid dispersing medium (mainly water) in the form of rod of about 0.01 ⁇ m ⁇ 0.1 ⁇ m (diameter ⁇ length) or in the form of fiber.
• the pH of the colloidal alumina is 3 to 5.
• the slurry obtained by adding such colloidal alumina to refractory particles shows a good fluidity when stirred, but shows a phenomenon that the viscosity of the slurry gradually increases when the stirring is stopped.
• colloidal alumina it is preferable to add the colloidal alumina to the refractory particles in an amount of 20 to 40% by weight based on the weight of the refractory particles. If the amount is less than 20% by weight, the fluidity of the slurry becomes worse, while if the amount is larger than 40% by weight, there is a tendency to bring about shrinkage of the mold produced, which undesirably results in worsening dimensional accuracy. If colloidal silica is used in place of colloidal alumina, or a mixture of colloidal alumina and colloidal silica is used, cracks are formed on the mold produced or sink mark on the mold surface; this was ascertained by experiments.
• a surface active agent to the slurry.
• the viscosity of the slurry can be controlled to be maintained low when stirred so as to maintain the slurry state excellent in fluidity.
• a surface active agent having the same pH as that of the colloidal alumina As an example, the use of anionic surface active agent is preferable.
• the pH of surface active agent is either higher or lower than the pH of the colloidal alumina, the effect of improving the fluidity of slurry is lessened.
• the surface active agent in an amount of 0.05 to 1% by weight based on the weight of the colloidal alumina. If the amount is too small, the effect of addition of the surface active agent cannot be obtained, while if the amount is too much, there easily takes place the formation of voids on the mold surface.
• the surface active agent is preferably added after the addition of the colloidal alumina to the refractory particles.
• the slurry is kneaded sufficiently, followed by filling in the pattern (cavity) having water absorption properties.
• the slurry filled in the pattern (cavity) having water absorption properties increases its viscosity gradually. Further the water in the slurry is absorbed by the pattern having water absorption properties, resulting in setting gradually from the surface portion of the mold.
• the surfaces of refractory particles seem to be coated with colloidal alumina coating layers, which bind individual refractory particles.
• the resulting solid is, then, fired.
• the mold is sometimes broken during the firing.
• the heating temperature for drying is preferably 100° C. or higher. In such a case, when the temperature is raised from room temperature to 100° C. or higher instantly, there is a fear for breaking the mold. Therefore, it is preferable to heat the mold initially at about 50° C., more preferably 30° to 60° C., for about 1 to 5 hours, followed by heating at between 100° C. and 250° C. The drying can be conducted while placing the resulting mold in the pattern (cavity) or after removing the mold from the pattern (cavity).
• Firing is conducted for removing water of hydration of colloidal alumina, and for removing unnecessary ingredients mixed in the course of production of the mold by heating the mold at a temperature at least as high as the temperature of a liquid metal when casting the liquid metal in the mold.
• the firing temperature should be at least the temperature of removing the water of hydration of colloidal alumina or higher. Since the temperature of removing the water of hydration of colloidal alumina is about 680° C., the firing temperature should be 680° C. or higher. Preferable firing temperature is a temperature higher than the pouring temperature of a liquid metal. Further, since the purpose of firing is not sintering of the refractory particles, too high a temperature is not necessary. Instead, since the mold after casting is required to be destructible and the refractory particles are required to be used repeatedly, it is preferable to make the firing temperature lower than the sintering temperature of the refractory particles.
• the firing can be conducted under an oxidizing atmosphere, such as air.
• the firing can be conducted while retaining the mold in the pattern (cavity) or after removing the mold from the pattern (cavity).
• the colloidal alumina becomes alumina.
• the resulting alumina functions as a binder for binding individual refractory particles and said effect as a binder does not deteriorate up to near 1600° C.
• the mold or core produced by the procedures mentioned above has excellent heat resistance and the shape thereof is not destroyed even if contacted with a liquid metal heated at 1200° to 1600° C. Further, cracks are not formed on the mold surface during the molding or pouring of a liquid metal. Further, since the shrinkage of the mold is very small, castings having high dimensional accuracy, more concretely within a dimensional variation of ⁇ 0.25 mm, can be produced.
• a core was produced by using zircon having a particle size of 37 to 53 ⁇ m (average 45 ⁇ m) as refractory particles and a plaster pattern as pattern having water absorption properties as follows.
• the resulting core was set in a predetermined place in a mold having a shape of a gas turbine bucket.
• Wax was poured around the core by using an injection molding machine to give a wax pattern having the shape of a gas turbine bucket.
• An oil was sprayed previously on the inner surface of the mold in order to improve mold release of wax.
• the pattern was equipped with a feeder head, a runner, a gate, and the like. The oils attached to the surface of the pattern were removed by cleaning using a mixed solution of acetone and an alcohol.
• the thus produced wax pattern was dipped in a slurry containing zircon particles and colloidal silica to attach the slurry around the pattern surface. Further, before the attached slurry was dried, molten quartz having a size of 100-150 mesh was sprinkled over the mold surface.
• the coating procedures of the wax mold surface with the slurry and the molten quartz were conducted in a constant temperature chamber at 25° C.
• the particle size of zircon in the slurry was 37 to 53 ⁇ m and the amount of colloidal silica was 40%.
• the thickness of the coating obtained by the slurry and the molten quartz was 0.2 to 0.3 mm in total.
• the procedures of attaching the slurry on the wax mold surface and sprinkling of quartz were repeated 10 times, respectively.
• the particle size of quartz was changed to 20 to 50 mesh from the sprinkling of the second time and thereafter.
• the wax was melted out in an autoclave under a pressure of 8-10 kg/cm 2 , followed by firing at 1000° C. for 2 hours to give the desired mold.
• nickel-base alloy was poured into the resulting mold.
• the composition of the nickel-base alloy was carbon 0.1%, silicon 0.30%, manganese 0.20%, aluminum 3.7%, cobalt 9.0%, chromium 16.3%, iron 0.5%, molybdenum 2.0%, balance being nickel and impurity elements contaminating at the time of melting.
• the melting and pouring of the nickel-base alloy was conducted in vacuum as follows.
• a vacuum vessel was divided into two, i.e. upper and lower chambers.
• a melting furnace was installed in the upper chamber.
• the mold was placed in the lower chamber and made removable to the upper chamber. Both the upper and lower chambers were kept in vacuum of 3 ⁇ 10 -4 torr.
• the nickel-base alloy was melted in the melting furnace in the upper chamber.
• the mold was placed in the lower chamber until the nickel-base alloy was melted, while the mold was heated at 1000° C. therein. After the nickel-base alloy was melted, the mold was moved to the upper chamber.
• the molten nickel-base alloy was poured into the mold at the pouring temperature of 1470° C. After the molten alloy was agglomerated in the mold, the mold was moved downwardly to the lower chamber and allowed to cool to room temperature as it was. Then the mold and the resulting casting was removed from the lower chamber and the mold was disjointed.
• the mold on the surface of the resulting casting was removed by sand blast and the core was removed by high-pressure jet water of 700 kg/cm 2 .
• the dimensional accuracy of the resulting gas turbine bucket was ⁇ 0.25 mm. Since the dimensional accuracy according to the process disclosed in Japanese Patent Appln Kokoku (Post-Exam Publn) No. 20848/63 wherein the hydrolyzed ethyl silicate solution is used to ⁇ 0.5 mm, the dimensional accuracy by using the core obtained by the process of this invention is remarkably high. Further, the surface appearance of the resulting gas turbine bucket was very smooth and sufficiently satisfactory.
• An outer mold (an outer mold consisting of cope and drag) was made as follows.
• a mixture of zircon having a particle size of 63-74 ⁇ m and zircon having a particle size of 105-150 ⁇ m in a weight ratio of 7:3 was prepared as refractory particles in an amount of 10 kg.
• As the binder a mixture of colloidal alumina and colloidal silica in a weight ratio of 6:4 was used in an amount of 600 g.
• a mixture of the above-mentioned refractory particles and this binder was used as the molding material and molded into the outer mold by compacting.
• the above-mentioned core was set. Then, the resulting mold was heated to give 800° C. in an electric furnace. When the mold temperature was raised to 600° C., a molten 13 chromium cast steel was poured into the mold at the pouring temperature of 1600° C. After cooling the mold, the resulting casting was removed therefrom and shaking out was conducted by shot blast. The core was removed by dissolving it by dipping in molten sodium hydroxide at 600° C. for 1 hour.
• composition of 13 chromium cast steel was carbon 0.3%, manganese 0.8%, silicon 1.0%, chromium 13.0%, nickel 0.6%, balance being iron and impurities.
• the casting was cut and the quality of inner portion was tested. There was no defect in the inner portion.
• the dimensional accuracy at the outlet portion of the pump impeller was ⁇ 0.25 mm and the surface roughness was 5-8 ⁇ m. Since the dimensional accuracy according to the process disclosed in Japanese Patent Appln Kokoku (Post-Exam Publn) No. 20848/63 wherein the hydrolyzed eithyl silicate solution is used is ⁇ 0.5 mm, and the surface roughness 15-35 ⁇ m, the casting obtained by using the core obtained by the process of this invention is excellent in dimensional accuracy and surface smoothness.
• Cores were produced in the same manner as described in Example 1 except for using colloidal silica in place of colloidal alumina, or using a mixture of colloidal silica and colloidal alumina in place of colloidal alumina in the slurry for producing the core. No complete cores were produced in individual cases, since cracks were produced in the cores.
• molds particularly cores, having good heat resistance, dimensional accuracy and surface smoothness can be produced easily.
• the molding materials are used in the form of a slurry, moldability is excellent and variations caused by workers are very slight.

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• Engineering & Computer Science (AREA)
• Chemical & Material Sciences (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Dispersion Chemistry (AREA)
• Mold Materials And Core Materials (AREA)
• Molds, Cores, And Manufacturing Methods Thereof (AREA)

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Citations (9)

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• 1982
  • 1982-08-06 JP JP57136264A patent/JPS5927749A/ja active Granted
• 1983
  • 1983-08-04 US US06/520,518 patent/US4605057A/en not_active Expired - Fee Related

Patent Citations (9)

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